Magnetic structures for loudspeakers of the electrodynamic type have an annular air gap in which the moving coil operates to drive a diaphragm. For maximum efficiency, the magnetic flux density in the air gap should be very high. Such magnetic structures commonly have a permanent magnet of cylindrical configuration with a cup-like annular return path element of soft magnetic material within which the permanent magnet is positioned, with one end of the permanent magnet in engagement with the center of the return path element and the other end of the magnet forming an air gap with an annular rim on the return path element. A pole piece, which may be made of soft magnetic material, can be provided on the top end of the permanent magnet to form the air gap with the return path element.
The cylindrical permanent magnet of the loudspeaker magnetic structure which has been described is normally magnetized in an axial direction by use of magnetizing apparatus which engages the opposite ends of the magnet, to provide axial flux lines within the magnet. However, when the magnetic structure is used, the magnetic flux path from the magnet to the air gap must curve within the permanent magnet and enter the air gap at a circumference of one end of the magnet. The permanent magnet with an axially magnetized flux path is inefficient to provide the curved flux path to the air gap, and this results in a reduction in the flux density in the air gap with respect to that which would be provided by a permanent magnet in which the flux lines are curved.
Although it has been proposed to construct a permanent magnet having a preferred direction of magnetization along a curved path within the magnetic material, such an anisotropic magnet must be constructed by a special process which is much more expensive than the normal process for constructing permanent magnet materials.